15.1.1 Water in soils

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Table 15.1


Figure 15.2 Clay soils are fine textured and have a greater water-holding capacity than do sandy soils that are composed of larger particles. Clay soils hold a greater amount of plant-available water between field capacity (Ψsoil about -0.02 MPa) and wilting point (Ψsoil about -1.5 MPa) than do sandy soils. In both situations, water is held with progressively stronger forces as the soil mass dries out. For a given soil water content, dissolved solutes would depress soil water potential further, and plants would encounter even greater difficulty in satisfying potential transpiration (Based on Slatyer and McIlroy 1961).

Water drains through soil due to gravity, leaving medium-sized pores still filled, and with thin films around soil particles. A soil in that state is at field capacity, and if soil surface evaporation plus plant transpiration (evapotranspiration) is prevented, that moisture will be retained indefinitely. In fine-textured soils (clays) soil particles and channels are small, and much of the water is effectively held by matric forces. Such soils do not drain freely and clay soils are able to store large amounts of water, which can be used by plants provided root length density is sufficiently high. In contrast, sandy soils have larger particles and larger channels. These soils are better aerated and drain freely, but store less water (Table 15.1).

Soil water potential (Ψsoil) declines as soil water content diminishes due to evapotranspiration, but the form of that relationship varies according to a soil’s physical properties (Figure 15.2). Clay and loam soils contain a greater amount of plant-available moisture within a given range of soil water potential than do sands, one reason why loam soils are favoured for agriculture. Plant-available moisture, that held between field capacity and wilting point (c. -0.01 to -1.5 MPa in Figure 15.2), is clearly greater in a clay soil than in a sandy soil, but root length density (length of roots per unit volume of soil) tends to be higher in lighter textured soils. As a result, plant-extractable moisture in clay soils is somewhat less than the soil’s physical properties alone would imply. In addition, hydraulic conductivity of clay soils decreases from about 10 metres per hour per megapascal (m h–1 MPa–1) at field capacity to around 0.1 m h–1 MPa–1 at wilting point. Put another way, water movement towards roots is greatly impaired as clay soils dry, while root growth towards sources of water is severely restrained by increasing soil strength. Despite their substantial moisture reserves, fine-textured (clay) soils are generally less hospitable to plant roots than loamy or sandy soils.